This invention relates to catalytic converters used to treat the exhaust emissions of internal combustion engines. More particularly, this invention relates to catalytic mufflers used on small internal combustion engines that power lawnmowers, snowblowers, generators and the like.
Recent governmental regulations require that the exhaust emissions of small internal combustion engines be substantially reduced. One way to reduce the exhaust emissions is to use a catalytic converter to treat the exhaust emissions of the engine. For small internal combustion engines, it is often desirable to combine the catalytic converter with a muffler into a single, compact unit.
A catalytic converter or a catalytic muffler represents a substantial increase in the total cost of the lawnmower, snowblower, or generator to which it is added. Therefore, it is desirable to decrease the cost of catalytic converters and catalytic mufflers to the extent possible.
Typical prior art catalytic converters have a venturi device that is substantially circular in the cross-sectional plane which is transverse to the direction of movement of the exhaust gases through it. Although these circular venturi devices are relatively efficient and effective in mixing secondary air with the exhaust gases, they are expensive to manufacture. Such venturi devices require a specially formed, separate piece of tubing or a machined part to yield a round venturi passageway. When catalytic converters and catalytic mufflers are made in large volumes, the savings achieved by eliminating this round venturi device are quite substantial.
Typical prior art catalytic mufflers have a noise attenuator that is disposed downstream of the catalyst. In these arrangements, undesirable back pressure is applied to the venturi, which minimizes secondary air induction and reduces the catalytic reaction. This back pressure reduces the throughput and thus the efficiency of the catalyst.